Storage tanks for holding a variety of fluids such as oil, gasoline, and diesel fuel are known in the art. An internal fuel tank on a marine vessel is an example of such a storage tank. Such a fuel tank may be provided with a vent to enable vapor and fumes to escape under pressure while fuel is being pumped into the fuel tank via a fuel fill tube. As the engine consumes fuel, air is drawn into the tank via the vent to fill the space from the consumed fuel. Venting also accommodates expansion of the fuel when it is heated. During filling of the fuel tank, some fuel may be discharged through the vent into the water if a person attempts to fill the tank to capacity.
The use of fuel dispensing nozzles that automatically shut off the flow of fuel to the tank when the tank is full have been used to avoid fuel spillage during filling. These nozzles may operate by sensing a pressure change at an end of the nozzle that results from fuel backing up within the tank fill tube.
A prior art nozzle 10 is illustrated in FIG. 1. Fuel is pressurized in the nozzle passage 1 by a pump (not shown). The flow of fuel is blocked in the nozzle 10 by a valve 3 that is held in a closed position by a spring 2. The valve 3 is connected to a hand-operated trigger 4 at a pivot point 5. The trigger 4 is also connected to a piston 6 at a second pivot point 7. The piston 6 is locked in a dispensing position by a pin 8 that forces balls 9 into a groove in the nozzle housing 11. The pin 8 is connected to a diaphragm 12 that is held in position by a second spring 13. When fuel is dispensed, the trigger 4 is lifted, lifting the pivot point 5 and the valve 3, allowing fuel to flow. The fuel travels to a venturi 15 where a spring loaded ball and seat create a vacuum in a passage 16 that is in communication with the diaphragm 12.
The passage 16 is also open to atmospheric pressure through an aspirator hole 17 near the end of the dispensing nozzle. When fuel is being dispensed, the pressure in the passage 16 is lowered by the venturi 15, but is replaced by atmospheric pressure through the aspirator hole 17 in the nozzle. During conventional automatic shut-off, when the aspirator hole 17 is covered by fuel surging up from the tank's fill tube, the pressure drops in the passage 16, drawing the diaphragm 12 against the second spring 13, and the pin 8 is lifted from its locking position. Thus, the piston 6 moves to release the pivot point 7 in the trigger. When the pivot point 7 is moved, the trigger 4 is ineffective and the spring 2 pushes the valve 3 into the closed position, stopping the flow of fuel.